Angle sensor

ABSTRACT

A two-axis angle sensor comprising a pair of bubble chambers ( 3,4 ) and associated bubble detectors ( 5,6 ). A processor ( 42 ) calculates the angle of the sensor by combining the signals from the first and second bubble detectors in accordance with a predetermined algorithm. A radiation source ( 1 ) illuminates a bubble ( 8 ) with radiation whereby the radiation is refracted by the bubble. A radiation detector ( 5,6 ) is positioned to detect the refracted radiation from the bubble and generate a signal indicative of the angle of the bubble chamber. An angle sensor is used to calibrate a vibrating structure gyroscope ( 70 ). A flywheel ( 91 ) is provided to generate a gyroscopic resistance to operator input. A method of manufacturing a bubble angle sensor is disclosed, the method employing a lid ( 61 ) with an internal recess ( 65 ).

The present invention relates to improvements in angle sensors.

In U.S. Pat. No. 5,794,355 a container is formed by a pair ofconcentrically aligned hemispherical surfaces. The container is filledwith a viscous fluid and a bubble of a lighter-weight fluid and placedbetween a radiation source and a radiation detector. The bubble changesposition within the container when the sensor is moved, transmitting abeam of radiation from the radiation source through the bubble toactivate a section of the radiation detector while the remainder of theradiation is blocked by the fluid. Two-axis angle sensing is achieved bythe use of a radiation detector comprising a two-dimensional array ofgrid elements (eg photodiodes).

The arrangement of U.S. Pat. No. 5,794,355 suffers a number of problems.Firstly, in order to achieve a measurement range of n measurement unitsthe two-dimensional radiation detector must have n*n grid elements. As nincreases the radiation detector can become large and expensive.Secondly, random measurement errors in the signal from the radiationdetector can be large.

An alternative method of achieving two-axis angle sensing is suggestedin U.S. Pat. No. 5,218,771 at column 4 lines 15-19. This suggests usingtwo angular motion detectors, mounted with their respective central axesoriented perpendicular to each other. However the arrangement of U.S.Pat. No. 5,218,771 still suffers from the problem of random measurementerror.

In accordance with a first aspect of the present invention there isprovided a two-axis angle sensor comprising

a first bubble chamber containing two fluids of differentcharacteristics such that a bubble is formed in the chamber;

a first bubble detector for generating a signal indicative of theorientation of the first bubble chamber with respect to a first detectoraxis by sensing the position of the bubble in the first bubble chamber;

a second bubble chamber containing two fluids of differentcharacteristics such that a bubble is formed in the chamber;

a second bubble detector for generating a signal indicative of theorientation of the second bubble chamber with respect to a seconddetector axis by sensing the position of the bubble in the second bubblechamber, and

a processor for calculating the angle of the sensor with respect tofirst and second measurement axes by combining the signals from thefirst and second bubble detectors in accordance with a predeterminedalgorithm, wherein the measurement axes are angularly offset from thedetector axes.

Instead of aligning the bubble detectors and bubble chambers with themeasurement axes and taking direct independent readings from the bubbledetectors, we offset the bubble detectors and bubble chambers from themeasurement axes and combine the signals from the bubble detectors. Wehave recognised that if the detector signals suffer from random errorswith a gaussian distribution then the error associated with one bubbledetector will tend to cancel out the error associated with the otherbubble detector. Thus the combined measurement will be more accuratethan a single independent measurement.

The bubble chambers and bubble detectors can then be provided in asensor housing which is aligned with the measurement axes. Thus forexample the sensor housing may comprise a joystick which is shaped to begripped by the hand such that the “forward/reverse pitch” direction isaligned with one measurement axis and the “left/right roll” direction isaligned with the other measurement axis. Alternatively the sensor may bea “mouse” type computer input device which is gripped by the hand butnot constrained to be used on a surface. In this case the sensor housingwill be shaped to be gripped by one or more hands such that the“forward/reverse” direction is aligned with one measurement axis and the“left/right roll” direction is aligned with the other measurement axis.One or more buttons may also be provided in a position to ensure thatthe sensor housing is gripped in the preferred orientation. In a furtheralternative the sensor may be mounted in a vehicle or aeroplane with oneof the measurement axes aligned with the direction of forward movement.

A variety of algorithms may be used, depending on the outputs of thedetectors. In a preferred example the predetermined algorithm comprises:

summing a pair of values derived from the bubble detector signals tocalculate the angle of the sensor with respect to the first measurementaxis; and

subtracting a pair of values derived from the bubble detector signals tocalculate. the angle of the sensor with respect to the secondmeasurement axis.

The precise form of the algorithm will depend on the angularrelationship of the axes. These axes may be offset from each other byany desired angle. However in a preferred example the detector axes arearranged substantially at right angles to each other, and themeasurement axes are arranged substantially at 45 degrees to thedetector axes. This enables the bubble detector signals to be simplyadded or subtracted without requiring either signal to be scaled up ordown with respect to the other signal before addition or subtraction.

A further problem with the arrangement of U.S. Pat. No. 5,794,355 isthat some radiation may be transmitted through the viscous fluid makingit difficult to detect the bubble. U.S. Pat. No. 5,794,355 addressesthis problem by adding a dye to the viscous fluid to absorb theradiation. In addition there is no design freedom in the positioning ofthe radiation detector—ie. it must be positioned directly behind thecontainer on the opposite side to the radiation source.

In U.S. Pat. No. 5,218,771 the materials forming the bubble and theliquid are chosen so that the interface surface between them is highlyreflective. Thus the detector detects light reflected from the surfaceof the bubble. A problem with this arrangement is that there is nodesign freedom in the positioning of the radiation detector—ie. it mustbe positioned to receive the reflected light from the bubble.

In accordance with a second aspect of the present invention there isprovided an angle sensor comprising

a bubble chamber containing two fluids of different characteristics suchthat a bubble is formed in the chamber;

a radiation source for illuminating the bubble with radiation wherebythe radiation is refracted by the bubble; and

a radiation detector positioned to detect the refracted radiation fromthe bubble and generate a signal indicative of the angle of the bubblechamber.

In contrast to the conventional approach we detect refracted radiationfrom the bubble. As a result it is not necessary to dye the fluid.Furthermore we can accommodate different arrangements for the radiationsource and radiation detector by selecting appropriate refractiveindices for the bubble chamber and the two fluids. This is not possiblein prior art systems which detect reflected or transmitted light sincethe angle of reflection or transmission is fixed regardless of therefractive indices.

In a preferred arrangement an interface is positioned to receive therefracted radiation from the bubble and deliver the refracted radiationto the radiation detector. This increases the amount of refractedradiation falling on the detector, thus improving the measurementaccuracy.

A number of different interfaces may be provided. For instance theinterface may comprise a light guide such as a fibre-optic cable. In onearrangement the interface comprises one or more lenses positionedbetween the bubble chamber and the radiation detector for focusing therefracted radiation onto the radiation detector. Alternatively theinterface may comprise a radiation transmissive projection in the bubblechamber. The projection typically has a face arranged at an angle suchthat refracted radiation from the bubble passes through the face, andradiation from other directions is internally reflected by the face backinto the bubble chamber.

In order to absorb radiation which has not been refracted by the bubble,the bubble chamber preferably has a radiation absorbent portion and aradiation transmissive window positioned adjacent to the bubble. Thismay be achieved by manufacturing the bubble chamber from two differentmaterials having different radiation absorption characteristics. Howeverin a preferred arrangement the radiation absorbent portion is formed bya coating of radiation absorbent material.

In the arrangement of U.S. Pat. No. 5,218,771 a single light source anda pair of light sensors are placed in a T-shaped arrangement with thesensors on opposite sides of the bubble chamber. Thus each sensordetects light which has been reflected to the right or to the left bythe bubble. The position of the bubble is deduced by analysing theoutputs of the sensors. A problem with this system is that a complexcalculation must be performed using a standard table lookup scheme todeduce the position of the bubble. Therefore in a preferred embodimentof the present invention the radiation detector comprises a positionsensitive detector, the position of the refracted radiation on thedetector being indicative of the position of the bubble in the bubblechamber. This provides a simpler method of position measurement thanU.S. Pat. No. 5,218,771.

Typically the position sensitive detector comprises an array ofdetectors, such as a charge-coupled device (CCD). Typically the array ofdetectors are arranged in a substantially straight line. If the bubblemoves along a curved path then preferably a cylinder lens is provided toproject the curved path onto a straight line detector.

In order to ensure that refracted radiation from the bubble falls ontothe detector it is important to arrange the radiation source at thecorrect position. In a preferred embodiment the radiation source ispositioned to illuminate the bubble with a beam of radiation which isoffset from the centre of the bubble. This arrangement ensures that therefracted output beam subtends the illuminating beam at an angle.Conveniently a radiation guide is provided for guiding radiation betweenthe radiation source and the bubble at the desired incident angle.

Typically the refractive index of the fluid forming the bubble is lowerthan the refractive index of the other fluid. In this arrangement thebubble causes the illuminating radiation to diverge, increasing the needfor one or more lenses for focusing the refracted radiation onto theradiation detector. Conveniently bubble comprises a gas bubble (althoughtwo liquids could also be used).

Conveniently the radiation source comprises an infrared radiationsource. This enables the radiation detector to be insensitive to strayvisible background optical radiation.

Where two or more angle sensors are used, as for example in accordancewith the first aspect of the invention, each may have its own radiationsource and detector or they may use common sources and/or detectors by,for example, operating at different frequencies depending upon thesensor concerned.

An inertial sensor known as a vibrating structure gyroscope (VSG) isdescribed in EP-A-0457541, JP-A-09050343 and “PS/2: GYROSCOPIC MOUSEDEVICE”, IBM TECHNICAL DISCLOSURE BULLETIN, vol. 34, no. 11, April 1,1992, pages 89-90. A common problem with VSGs is the need to manuallyset the device or provide a fixed or controlled horizon.

In accordance with a third aspect of the present invention there isprovided an angle sensor comprising

a vibrating structure gyroscope (VSG) for generating a VSG signalindicative of the orientation of the sensor;

a bubble chamber containing two fluids of different characteristics suchthat a bubble is formed in the chamber;

a bubble detector for generating a calibration signal responsive to theposition of the bubble in the bubble chamber; and

a processor for calibrating the VSG signal from the calibration signalin accordance with a predetermined calibration algorithm.

This provides a robust and cost effective method of calibrating the VSGsignal.

Any calibration algorithm may be used but in a preferred arrangement thebubble detector generates the calibration signal when the bubble passesa centre point. The calibration signal thus provides an artificialhorizon to calibrate the VSG signal.

A further problem with conventional angle sensors is that the sensordoes not provide any resistance to operator input.

In accordance with a fourth aspect of the present invention there isprovided an angle sensor comprising

means for generating a signal indicative of the orientation of thesensor; and

a flywheel which provides a gyroscopic force to oppose tilting of thesensor.

The flywheel creates the effect of a stable platform which increasesoperator feedback and dampens over-enthusiastic input from the operator.In a preferred arrangement means are provided to vary the speed ofrotation of the flywheel in order to vary the gyroscopic force, forinstance in conjunction with a game being played using the angle sensoras an input device.

The means for generating a signal indicative of the orientation of thesensor may comprise a VSG. However in a preferred arrangement the meansfor generating a signal indicative of the orientation of the sensorcomprises:

a bubble chamber containing two fluids of different characteristics suchthat a bubble is formed in the chamber; and

a bubble detector for generating a signal indicative of the orientationof the first bubble chamber by sensing the position of the bubble in thebubble chamber.

Typically the flywheel is driven by a motor, for example a DC or threephase AC motor.

A further problem associated with bubble angle sensors is that it can bedifficult to ensure that the bubble is of the correct size. If thebubble is too large or too small the accuracy of the sensor may becompromised.

In accordance with a fifth aspect of the present invention there isprovided a method of manufacturing an angle sensor, the methodcomprising:

providing a container with an open end;

substantially filling the container with liquid;

providing a lid with one or more internal recesses;

closing the liquid filled container with the lid to form an enclosedbubble chamber, wherein on closing the container a gas bubble becomestrapped in the or each internal recess; and

arranging a bubble detector to generate a signal indicative of theorientation of the bubble chamber by sensing the position of the gasbubble in the bubble chamber.

A number of recesses may be provided but in a preferred arrangement onlya single recess is provided, the recess having a volume correspondingwith the volume of bubble required. Typically the lid is pivotallyattached to the container, and rotated to close the container.

In all these aspects of the invention, the bubble chamber preferably hasa D-shape but it could also be circular or take some other convenientform.

The angle sensor according to the first, second, third, fourth and fifthaspects of the present invention may be employed in a variety ofapplications. However in a preferred example the angle sensor isincorporated in a user input device, for instance for a computer gameconsole or a PC. The input device may comprise a joystick which iscontrolled by one hand, or a “mouse” type input device which is held inboth hands and rotated in free space. Alternatively the angle sensor maybe used to detect the orientation of a vehicle or aeroplane.

A number of embodiments of the present invention will now be describedwith reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view taken along line B in FIG. 2;

FIG. 2 is a plan view of a two-axis angle sensor;

FIG. 3 is a cross-section taken along line C in FIG. 2;

FIG. 4 is a schematic perspective view of the two-axis angle sensor;

FIG. 5 is a cross-section through part of the bubble chamber showing theoptical path of the illuminating radiation where no bubble is present;

FIG. 6 is a cross-section through the bubble chamber showing theradiation beam refracted by the bubble;

FIG. 7 is a schematic diagram of the processing electronics of thetwo-axis angle sensor;

FIGS. 8-10 are schematic plan views of the two-axis angle sensor of FIG.2;

FIG. 11 is a side view of the first stage of manufacture of a bubblechamber;

FIG. 12 is a cross-section through the centre of the bubble chamberafter the lid has been closed;

FIG. 13 is a cross-section along line D in FIG. 12;

FIG. 14 is a cross-section along line E in FIG. 12;

FIG. 15 is a schematic view of an alternative angle sensor incorporatinga VSG;

FIG. 16 illustrates a calibration routine implemented by themicroprocessor;

FIG. 17 is a schematic illustration of an alternative angle sensorincorporating a fly-wheel;

FIG. 18 is a perspective view of part of an alternative two-axis anglesensor; and

FIG. 19 is a plan view of a user input device.

Referring to FIGS. 1-4, a pair of infra-red point light sources 1, 2each illuminate a respective transparent cast acrylic resin bubblechamber 3, 4 with infra-red radiation. Radiation from the bubble chamber3 is imaged onto an infra-red CCD line sensor 5 and radiation from thebubble chamber 4 is imaged onto an infra-red CCD line sensor 6. Theimaging optics for each bubble chamber 3, 4 is identical and the opticsfor bubble chamber 3 is shown in detail in FIG. 1.

The bubble chamber 3 has a D-shaped internal cavity with a semicircularupper surface 9 and a planar lower surface 10. The cavity is partiallyfilled with a first fluid, ethanol 7, to leave a bubble 8 of a secondfluid, air, which floats at the top of the cavity against the uppersurface 9. As shown in FIG. 1, the D-shaped form of the cavity ensuresthat the bubble 8 engages the upper surface 9 but does not engage thelower surface 10. As a result the bubble 8 moves easily along an arcuatepath 31 adjacent the curved upper surface 9 as the bubble chamber 3 isrotated. Of course other combinations of fluids such as water and air ortwo immiscible liquids could be used.

Infra-red radiation from the source 1 is guided towards the path 31 by aconically shaped guide 11 (shown clearly in the perspective view of FIG.4) with parallel sides 12, 13. A projection 14 is also provided on theoutput side of the bubble chamber 3.

The principle optical modes of the sensor are shown in FIGS. 5 and 6.For the purposes of clarity, hashed cross-section lines are not includedin FIGS. 5 and 6. As shown in FIG. 5, with no bubble present radiation15 from the light source 1 passes substantially unrefracted through thebubble chamber 3 (apart from a small amount of refraction at theacrylic/ethanol interfaces) With a bubble present the illuminating beamof radiation is refracted downwards by the bubble as shown in FIG. 5 anddiscussed in detail below.

The parallel sides 12, 13 of the light guide 11 subtend an angle of 43°with a horizontal line 16. As a result, an illuminating beam ofradiation from the light source 1 is directed upwards at an angle of 43°towards the bubble as indicated by a ray 17. The illuminating beam isrefracted slightly downwards at the acrylic/ethanal interface asindicated by a ray 18. The ray 18 (which is at the centre of theilluminating beam) is offset from the centre 33 of the bubble 8. As aresult the illuminating beam is refracted downwards at the ethanol/airinterface as indicated by a ray 19. The angle of refraction isrelatively large due to the large change in refractive index at theethanol/air interface. If necessary the angle of refraction can beadjusted by choosing different fluids. The radiation is then refracteddownwards further at the air/ethanol interface as indicated by a ray 20.Finally the radiation is refracted slightly at the ethanol/acrylicinterface to give an output beam indicated by ray 24.

The refractive indices of the fluids and the size of the bubble 8 arechosen so that the output beam is directed towards the projection 14 andsubtends an angle of 43° with the horizontal line 16. In one example theacrylic forming the bubble chamber 3 has a refractive index of 1.49, theethanol has a refractive index of 1.36 and the air bubble 8 has arefractive index of approximately 1.00.

The projection 14 has an upper face 21 which subtends an angle of 43°with the horizontal line 16. The projection 14 also has an angled outputface 23 which is substantially perpendicular to the output beam. Thisensures that the output beam (as illustrated by ray 24) is transmittedby the output face 23 without being reflected.

Whilst the primary optical path of the illuminating radiation isillustrated in FIG. 5, some of the radiation will follow other opticalpaths. For instance some radiation will be reflected by the far wall ofthe bubble and scattered back into the bubble. This results in an evenbackground illumination. A D-shaped optically absorbent coating 25, 26(shown in FIGS. 3, 5 and 6) is provided on the front and rear walls ofthe cavity in the lower part of the chamber in order to attenuate thisbackground illumination. In addition any stray radiation which falls onthe angled output face 23 at an angle less than the acrylic/air criticalangle of 42° will be reflected back into the bubble chamber.

Referring back to FIGS. 1 and 2, a wide-angle plano-concave lens 27 anda pair of double convex lenses 28, 29 image the radiation from theprojection 14 to a flat plane (not shown). Two double convex lenses 28,29 are used as the required focal length is short and this is morereadily obtained using a pair of lenses. A cylinder lens 30 modifies theoptical path so that the semicircular path 31 is imaged onto thestraight line sensor 5 irrespective of the change in vertical positionas the bubble 8 moves in the semicircular arc. The cylinder lens 30 hasthe additional function of expanding the apparent movement of theradiation from the bubble as the bubble traverses towards the moreextreme limits of travel. As it is moving along the path 31 the bubble'smotion in the horizontal is proportionally reduced, but the effect ofthe cylinder lens 30 is to expand this to a more linear displacementalong the line sensor 5.

The line sensors 5, 6 are conveniently mounted on respective circuitboards 34, 35. The optical axis of the lenses is optimally aligned withthe centre of the path 31 of the bubble in the chamber.

Referring to FIG. 7, the detection signals 40, 41 from the CCD linesensors 5, 6 are fed to a single chip micro-controller 42 which executesan internally stored programme. The micro-controller 42 converts theanalogue detection signals 40, 41 for each pixel of each line sensor 5,6 into a digital value using the micro-controller's onboard analogue todigital converter. The microcontroller 42 also performs ancillaryfunctions—such as checking for key switch closures from a keypad array43, displaying user selected configurations with a symbol display 44,and storing or retrieving configuration set-ups and/or calibration datain a non-volatile storage device 45.

The micro-controller 42 calculates the angle of the sensor andcommunicates the angle of the sensor (and any other data) to a hostcontroller 46 (such as a computer game console or PC). The angle iscalculated with respect to perpendicular measurement axes X,Y (shown inFIGS. 2 and 4) using the algorithm discussed below with reference toFIGS. 8-10.

In the schematic representations of FIGS. 8-10, the two line sensors 5,6are shown with +/− positions in the same sense as the bubble chambers3,4. In practice bubble displacement is detected as a movement in thedirection opposite to the bubble movement due to the inversion of theimage by the lenses 27-30. This inversion is accounted for in practicalimplementations but is ignored in FIGS. 8-10 for the sake of clarity.

The line sensors 5,6 are aligned with perpendicular detector axes A,B at45° to the X and Y measurement axes.

In FIG. 8 the sensor has been rotated clockwise about the Y axis asindicated at 50. As a result, the bubble 8 (and also the correspondingbright spot 8′ on the line sensor 5) has moved in the positive Adirection. The bubble 51 (and associated bright spot 51′ on line sensor6) has moved in the negative B direction.

In FIG. 9 the sensor has been rotated anti-clockwise about the X axis asindicated at 52. Bubble 8 and bubble 51 have moved in the negative A andB directions respectively.

In FIG. 10 the sensor has been rotated clockwise about the Y axis asindicated at 50 and anti-clockwise about the X axis as indicated at 52.As a result the bubble 8 has moved back to the centre of the chamberwhilst the bubble 51 has moved towards the negative B end of thechamber.

The microcontroller 42 thus implements the following algorithm, where:

Bubble position at Center=50

Full positive movement=101

Full negative movement=0

A=position of bubble 8 in direction A

B=position of bubble 51 in direction B

θ_(y) =A−B

E.g. for A=50, B=50 (center about y axis)

θ_(y)=50−50=0

E.g. for A=70, B=30 (right roll about y-axis)

θ_(y)=70−30=40

E.g. for A=101, B=0 (full right roll about y-axis)

θ_(y)=101−0=101

E.g. for A=35, B=65 (left roll about y-axis)

θ_(y)=35−65=−30

E.g. for A=0, B=101 (Full left roll about y-axis)

θ_(y)=0−101=−101

θ_(x)=((Center−A)+(Center−B))

E.g. for A=50, B=50 (center about x-axis)

θ_(x)=((50−50)+(50−50))=0

E.g. for A=30, B=30 (forward pitch about x-axis)

θ=((50−30)+(50−30))=40

E.g. for A=65, B=65 (reverse pitch about x-axis)

 θ_(x)=((50−65)+(50−65))=−30.

The values θ_(x) and θ_(y) are output by the micro-controller 42 to thehost 46 and can be used (for example) to issue commands to a computergame being executed by the host 46.

A method of manufacturing one of the bubble chambers 3,4 will now bedescribed with reference to FIGS. 11-14. The acrylic bubble chamber isformed as an open D-shaped container 60 with a flange 62 extendingaround its lower edge. A lid 61 is attached to the container 60 by asprung hinge 63. The lid 61 also has a domed recess 65 and a flexibleU-shaped locking member 64. The container is filled to the brim withethanol and then the lid 61 is closed and snapped shut with the flange62 received in the U-shaped locking member 64 as shown in FIGS. 12-14.This leaves an air bubble with a volume approximately equal to thevolume of the recess 65 in the bubble chamber. More than one recess 65may be formed in the lid 61 if required to make a larger bubble.

In the alternative embodiment of FIG. 15, a digital vibrating structuregyroscope 70 (such as the ENC-05E sensor manufactured by Murata)generates an electronic signal indicating the amount and direction ofmovement in degrees about the X-axis. A cylindrical bubble chamber 71contains two fluids of different characteristics such that a bubble 72is formed in the chamber 71. A light emitting diode 73 emits a narrowbeam of light through the centre of the bubble chamber 71 towards alight sensor 74. As the bubble 72 passes the centre point it is detectedby the sensor 74.

A microcontroller 75 implements the calibration routine shown in FIG.16. The microcontroller reads the VSG signal at 76 and checks theposition of the bubble at 77. If the bubble is at the centre position(78) then the signal from the VSG is calibrated to zero at 79. Thecalibrated data is then returned to a CPU 81 at 80. Two of the devicesof FIG. 15 can be positioned to create a two-axis angle sensor of thetype shown in FIG. 1.

In a further alternative embodiment shown in FIG. 17, an angle sensor 90(for instance the bubble angle sensor of FIG. 1 or the VSG angle sensorof FIG. 15) generates a signal indicative of the orientation of thesensor. A flywheel 91 (which is preferably in the form of an ACsynchronous motor) is rotated about the Z axis to create a gyroscopicforce when the sensor 90 is banked to one side about the X axis. Thisresistance creates the effect of a stable platform which can increaseoperator feedback and gaming pleasure and dampens over enthusiasticinput from the operator.

A controller 92 is provided to alter the speed of rotation of theflywheel 91 in conjunction with a particular game being played. Thuswhen the flywheel 91 is speeded up the operator finds it more difficultto bank the sensor, and when the flywheel 91 is slowed down it becomeseasier to bank the sensor.

In another application, a vibration effect can be achieved by repeatedlyand quickly speeding up and slowing down the flywheel.

FIG. 18 is a perspective view of an alternative angle sensor. A curvedtubular bubble chamber 100 is aligned with axis A and contains an airbubble 101 which moves along the bubble chamber 100 when the bubblechamber is rotated about the detector axis B. Five light emitting diodes102-106 are arranged with equal spacing on one side of the bubblechamber 100. Five light detectors 107-111 are arranged on the oppositeside of the bubble chamber 100. Thus five consecutive positions of thebubble 101 can be detected. In this example the sensor is convenientlyarranged so that the light emitting diodes 102-106 and detectors 107-111can share their information, creating twice as many incrementations asthe number of detectors. A second bubble chamber (not shown) and asecond set of diodes/detectors (not shown) is aligned with the B-axis togive a two-axis sensor similar to the sensor of FIG. 1. The signals fromthe ten detectors are combined to calculate the orientation of thesensor with respect to measurement axes X,Y.

FIG. 19 is a plan view of a computer game console input device 90housing an angle sensor (not shown). Any one of the sensors shown inFIGS. 1-10, 15, 17 or 18 may be used. As shown in FIG. 19, the sensorhousing 91 is provided with a set of buttons 92 and a cursor controller93. The sensor (not shown) is arranged with its detector axes A,B andmeasurement axes X,Y as shown in FIG. 19. The input device 90 isdesigned to be held by both hands with the buttons 92 being operated bythe right thumb, and the cursor controller 93 being operated by the leftthumb. This ensures that the Y measurement axis points directly forwardas viewed by the operator. The housing 91 may also be shaped suitably(for instance it may be rectangular or triangular in plan) to ensurethat the input device is held in the correct orientation.

The input device 90 is connected, in use, to a computer game console orPC via a flexible cable (not shown). Thus control of a game is achievedby holding the device in free space and tilting the device to the leftor right to issue a move left or move right command.

What is claimed is:
 1. A two-axis angle sensor comprising a first bubblechamber containing two fluids of different characteristics such that abubble is formed in the chamber; a first bubble detector for generatinga signal indicative of the orientation of the first bubble chamber withrespect to a first detector axis by sensing the position of the bubblein the first bubble chamber; a second bubble chamber containing twofluids of different characteristics such that a bubble is formed in thechamber; a second bubble detector for generating a signal indicative ofthe orientation of the second bubble chamber with respect to a seconddetector axis by sensing the position of the bubble in the second bubblechamber; and a processor for calculating the angle of the sensor withrespect to first and second measurement axes by combining the signalsfrom the first and second bubble detectors in accordance with apredetermined algorithm, wherein the measurement axes are angularlyoffset from the detector axes.
 2. A sensor according to claim 1 whereinthe predetermined algorithm comprises: summing a pair of values derivedfrom the bubble detector signals to calculate the angle of the sensorwith respect to the first measurement axis; and subtracting a pair ofvalues derived from the bubble detector signals to calculate the angleof the sensor with respect to the second measurement axis.
 3. A sensoraccording to claim 1 wherein the detector axes are arrangedsubstantially at right angles to each other.
 4. A sensor according toclaim 3 wherein the detector axes are arranged substantially at 45degrees to the measurement axes.
 5. A sensor according to claim 1further comprising a sensor housing for housing the bubble chambers andbubble detectors, wherein the sensor housing is aligned with themeasurement axes.
 6. A sensor according to claim 1 wherein the or eachbubble chamber is shaped such that the bubble only engages an uppersurface of the chamber.
 7. A sensor according to claim 1 wherein the oreach bubble chamber is substantially D-shaped.
 8. An angle sensoraccording to claim 1 comprising a flywheel which provides a gyroscopicforce to oppose tilting of the sensor.
 9. A sensor according to claim 8wherein the flywheel is rotated by an electric motor.
 10. A user inputdevice comprising a sensor according to claim
 1. 11. An angle sensorcomprising a bubble chamber containing two fluids of differentcharacteristics such that a bubble is formed in the chamber; a radiationsource for illuminating the bubble with radiation whereby the radiationis refracted by the bubble; and a radiation detector positioned todetect radiation refracted by the bubble and generate a signalindicative of the angle of the bubble chamber.
 12. A sensor according toclaim 11 further comprising an interface positioned to receive therefracted radiation from the bubble and deliver the refracted radiationto the radiation detector.
 13. A sensor according to claim 12 whereinthe interface comprises one or more lenses positioned between the bubblechamber and the radiation detector for focusing the refracted radiationonto the radiation detector.
 14. A sensor according to claim 13 whereinthe lens or one of the lenses comprises a cylinder lens.
 15. A sensoraccording to claim 12 wherein the interface comprises a radiationtransmissive projection in the bubble chamber.
 16. A sensor according toclaim 11 wherein the bubble chamber has a radiation absorbent portionand a radiation transmissive window positioned adjacent to the bubble.17. A sensor according to claim 16 wherein the radiation absorbentportion is formed by a coating of radiation absorbent material.
 18. Asensor according to claim 11 wherein the radiation detector comprises aposition sensitive detector, the position of the refracted radiation onthe detector being indicative of the position of the bubble in thebubble chamber.
 19. A sensor according to claim 18 wherein the positionsensitive detector comprises an array of detectors.
 20. A sensoraccording to claim 19 wherein the array of detectors are arranged in asubstantially straight line.
 21. A sensor according to claim 11 furthercomprising a radiation guide for guiding radiation between the radiationsource and the bubble.
 22. A sensor according to claim 11 wherein theradiation source is positioned to illuminate the bubble with a beam ofradiation which is offset from the centre of the bubble.
 23. A sensoraccording to claim 11 wherein the refractive index of the fluid formingthe bubble is lower than the refractive index of the other fluid.
 24. Asensor according to claim 11 wherein the radiation source comprises aninfra-red radiation source.
 25. An angle sensor according to claim 6comprising a vibrating structure gyroscope (VSG) for generating a VSGsignal indicative of the orientation of the sensor; and a processor forcalibrating the VSG signal from the calibration signal in accordancewith a predetermined calibration algorithm.
 26. An angle sensorcomprising: a bubble chamber containing a liquid and a gas bubble; and abubble detector for generating a signal indicative of the orientation ofthe bubble chamber by sensing the position of the gas bubble in thebubble chamber, wherein the bubble chamber has one or more internalrecesses having a total volume substantially equal to the volume of thegas bubble.
 27. A method of angle detection, the method comprising: (1)providing an angle sensor comprising first and second bubble chamberseach containing two fluids of different characteristics such that abubble is formed in each chamber; (2) generating a first signalindicative of the orientation of the first bubble chamber with respectto a first detector axis by sensing the position of the bubble in thefirst bubble chamber; (3) generating a second signal indicative of theorientation of the second bubble chamber with respect to a seconddetector axis by sensing the position of the bubble in the second bubblechamber; and (4) calculating the angle of the sensor with respect tofirst and second measurement axes by combining the first and secondsignals in accordance with a predetermined algorithm wherein thedetector axes are angularly offset from the measurement axes.
 28. Amethod of angle detection, the method comprising providing a bubblechamber containing two fluids of different characteristics such that abubble is formed in the chamber; illuminating the bubble with radiationwhereby the radiation is refracted by the bubble; and detectingradiation refracted by the bubble to generate a signal indicative of theangle of the bubble chamber.
 29. A method of manufacturing an anglesensor, the method comprising: providing a container with an open end;substantially filling the container with liquid; providing a lid withone or more internal recesses; closing the liquid filled container withthe lid to form an enclosed bubble chamber, wherein on closing thecontainer a gas bubble becomes trapped in the or each internal recess;and arranging a bubble detector to generate a signal indicative of theorientation of the bubble chamber by sensing the position of the gasbubble in the bubble chamber.